Integrated acoustic transducer in mems technology, and manufacturing process thereof

ABSTRACT

A MEMS acoustic transducer, for example, a microphone, includes a substrate provided with a cavity, a supporting structure, fixed to the substrate, a membrane having a perimetral edge and a centroid, suspended above the cavity and fixed to the substrate the membrane configured to oscillate via the supporting structure. The supporting structure includes a plurality of anchorage elements fixed to the membrane, and each anchorage element is coupled to a respective portion of the membrane between the centroid and the perimetral edge of the membrane.

BACKGROUND

1. Technical Field

The present disclosure relates to an integrated acoustic transducer inMEMS technology and to the manufacturing process, and in particular to amicro-electromechanical (MEMS) microphone of a capacitive type with asuspended-membrane mobile electrode and reduced residual stresses.

2. Description of the Related Art

As is known, an acoustic transducer, for example, a MEMS microphone, ofa capacitive type generally comprises a mobile electrode, in the form ofa diaphragm or membrane, arranged facing a fixed electrode, to providethe plates of a capacitor. The mobile electrode is generally anchored,by means of a perimetral portion, to a substrate, while a centralportion is free to move or bend in response to a sound-wave pressureacting on a surface of the mobile electrode. Since the mobile electrodeand the fixed electrode form the capacitor, bending of the membrane thatconstitutes the mobile electrode causes a variation of capacitance ofthe capacitor. In use, said variation of capacitance is converted intoan electrical signal, supplied as an output signal of the MEMSmicrophone.

As an alternative to MEMS microphones of a capacitive type, MEMSmicrophones are known, in which the movement of the membrane is detectedby means of elements of a piezoresistive, piezoelectric, or opticaltype, or also exploiting the tunnel effect.

MEMS microphones of a known type are, however, subject to problemsderiving from residual (compressive or tensile) stresses internal to thelayer that forms the membrane. The factors that affect stress aremultiple, and are due, for example, to the properties of the materialsused, to the techniques of deposition of said materials, to theconditions (temperature, pressure, etc.) at which deposition is made,and to possible subsequent thermal treatments.

Residual stresses are frequently the cause of mechanical deformations ofthe membrane, such as warping or buckling, and can significantly affectthe performance of the MEMS microphone by reducing the sensitivity.

Even though it is possible to control partially the amount of residualstress in the membrane by means of an appropriate design of the membraneitself and by evaluating the optimal manufacturing conditions, theresult obtained is not satisfactory for applications in which a highsensitivity is required. In these cases, in fact, the mechanicalbehavior in response to sound-wave stresses is in any case dominated bythe level of residual stress in the membrane.

To overcome these problems, described in WO 2008/103672 is a MEMSmicrophone of a capacitive type in which the mobile electrode (withmembrane of a circular shape) is suspended over a cavity by means of asingle anchorage element fixed with respect to a supporting beamprovided in the same layer in which the fixed electrode is formed. Thepoint of coupling of the anchorage element with the mobile electrode islocated in the center of the membrane that forms the mobile electrode.In this way, the mobile electrode can release the residual stressesthrough free radial contractions or expansions.

However, membrane mobile electrodes suspended to the fixed electrode bymeans of a central anchorage are readily susceptible, during use, toundesirable modes of pitch and roll, which cause a degradation of theperformance of the MEMS microphone that uses said mobile electrodes.

BRIEF SUMMARY

The present disclosure is to provide an integrated acoustic transducerin MEMS technology and a manufacturing process.

One embodiment of the present disclosure is a MEMS acoustic transducerthat includes a substrate having a cavity, a supporting structure fixedto the substrate; and a membrane having a perimetral edge and acentroid. The membrane is suspended above the cavity and fixed to thesubstrate and configured to oscillate through the supporting structure,wherein the supporting structure comprises a plurality of anchorageelements fixed to the membrane, each anchorage element being coupled toa respective portion of the membrane between the centroid and theperimetral edge of the membrane.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

For a better understanding of the present disclosure, embodiments arenow described, purely by way of non-limiting example, with reference tothe attached drawings, wherein:

FIG. 1 shows a top plan view of an assemblage of a rigid plate of anacoustic transducer according to one embodiment of the presentdisclosure;

FIG. 2 shows a cross-sectional view of an assemblage of a rigid plateand a mobile membrane of the acoustic transducer of FIG. 1, along a lineof cross section II-II of FIG. 1;

FIGS. 3-8 show a cross-sectional view of one half of the assemblage ofFIG. 2 during successive manufacturing steps;

FIG. 9 shows a top plan view of an assemblage of a rigid plate of anacoustic transducer according to another embodiment of the presentdisclosure;

FIG. 10 shows a cross-sectional view of an assemblage of a rigid plateand a mobile membrane of the acoustic transducer of FIG. 9, along a lineof cross section X-X of FIG. 9;

FIG. 11 shows a perspective view of the assemblage of FIG. 10;

FIG. 12 shows a top plan view of an assemblage of a rigid plate of anacoustic transducer according to a further embodiment of the presentdisclosure;

FIG. 13 shows a cross-sectional view of an assemblage of a rigid plateand a mobile membrane of the acoustic transducer of FIG. 12, along aline of cross section XIII-XIII of FIG. 12;

FIG. 14 shows a perspective view of the assemblage of FIG. 13; and

FIG. 15 shows a device that uses an acoustic transducer according to oneof the embodiments of the present disclosure.

DETAILED DESCRIPTION

FIGS. 1 and 2 show, respectively, a top plan view and a cross-sectionalview of an assemblage of a membrane and a rigid plate of an integratedacoustic transducer in MEMS technology, for example, amicroelectromechanical (MEMS) microphone, according to one embodiment ofthe present disclosure. For reasons of simplicity, in what followsreference will be made to said assemblage generally as a MEMS microphone1, even though the electronics of supply and conditioning of thesignals, which are, however, necessary, are not illustrated, and eventhough the description, albeit valid for any acoustic transducer, islimited in particular to a microphone.

With joint reference to FIGS. 1 and 2, the MEMS microphone 1 is amicrophone of a capacitive type and comprises a membrane 2, which ismobile and faces a rigid plate 3 (back plate), which is fixed. Themembrane 2 is suspended above a cavity 5 and is supported by one or moresupporting beams 7 (only one of which is shown in the figure) via aplurality of supporting elements 6, coupled to a respective portion ofthe membrane 2 comprised between the center 2′ and a perimetral edge 2″of the membrane 2. As may be seen in FIG. 1, the supporting beam 7 maycomprise a beam-bearing portion 7 a and a plurality of beam-appendageportions 7 b, fixed with respect to the beam-bearing portion 7 a. Thesupporting elements 6 can be provided either between the beam-appendageportions 7 b and the membrane 2 or between the beam-bearing portion 7 aand the membrane 2. Preferably, the supporting beam 7 is formed in thesame layer in which the rigid plate 3 is formed and is separated fromthe rigid plate 3 by means of a notch 9, formed, for example, by meansof techniques of chemical etching. In this way, the rigid plate 3 isdivided by the supporting beam 7 into two rigid-plate regions 3′, whichare lateral with respect to the supporting beam 7 itself. The tworigid-plate regions 3′ can be electrically connected to one another, forexample, by means of a connection path 13, made of conductive material.

Furthermore, the rigid plate 3, the supporting beam 7, the supportingelements 6, and the membrane 2 are advantageously made of the sameconductive material, for example, doped polysilicon, thus simplifyingthe manufacturing process and eliminating any possible problems due tonon-adhesion of the supporting elements 6 to the supporting beam 7 andto the diaphragm 2.

The rigid plate 3 can comprise a plurality of holes 8, of any path,preferably circular, having the function of favoring, during themanufacturing steps, removal of underlying layers (as will be explainedmore clearly in what follows) and, in use, of enabling free circulationof air between the rigid plate 3 and the membrane 2, thus reducing theeffect of squeeze-film damping. For the same reasons, also thesupporting beam 7 can comprise a plurality of holes 8.

The rigid plate 3 and the supporting beam 7 are anchored to a substrate10 via respective plate-anchorage portions 11 and beam-anchorageportions 12.

The plate-anchorage portions 11 preferably comprise peripheral areas ofthe rigid plate 3, which include, for example, an entire boundary of therigid plate 3, and are insulated from the substrate 10 by one or moreinsulating layers, for example, as illustrated in FIG. 2, by a firstinsulating layer 15 and by a second insulating layer 16. In addition tothe function of insulation of the substrate 10 from the rigid plate 3,the first and second insulating layers 15, 16 have, respectively, thefunction, during the manufacturing steps, of separating the membrane 2from the substrate 10, and of enabling separation between the rigidplate 3 and the membrane 2.

The beam-anchorage portions 12 preferably comprise two or more mutuallyopposite sides of the supporting beam 7 and are separated from thesubstrate 10 by one or more insulating layers, for example, by the firstinsulating layer 15 and by the second insulating layer 16, in a waysimilar to what is illustrated with reference to the plate-anchorageportions 11.

The supporting beam 7 and the membrane 2 are mechanically andelectrically connected to one another via the supporting elements 6, andinsulated from the rigid plate 3. Consequently, the membrane 2 and therigid plate 3 form plates of a capacitor, and the air that flows betweenthe membrane 2 and the rigid plate 3 through the holes 8 forms thedielectric arranged between the electrodes of the capacitor. In order toimplement the operations of a microphone of a capacitive type, the rigidplate 3 and the membrane 2 must be appropriately biased, while thesubstrate 10 supplies a ground reference signal. For this purpose, thereare provided: a first pad 20, made of conductive material and inelectrical contact with the membrane 2, preferably placed on thesupporting beam 7 for biasing the membrane 2 itself; a second pad 21,made of conductive material, placed in direct contact with the rigidplate 3, for biasing the rigid plate 3 itself; and a third pad 23, alsomade of conductive material, placed in electrical contact with a groundcontact 22, which is, in turn, in direct contact with the substrate 10,for collecting the ground reference signal.

In use, the cavity 5 has the function of acoustic input port, to enablesound-pressure waves 25, represented schematically in the figure asarrows, to enter and dynamically deform the membrane 2. As analternative, in a way not shown in the figure, the sound-pressure waves25 can reach the membrane 2 through the holes 8.

Since the membrane 2 is not anchored along its perimetral edge 2″, theresidual stresses of the membrane 2 are released through free radialcontractions or expansions. Furthermore, thanks to the presence of theplurality of supporting elements 6, the membrane 2 is not verysusceptible to detection errors due to undesired modes of pitch androll.

The respective stiffnesses of the rigid plate 3, of the supporting beam7, and of the membrane 2 can be used to vary the characteristics ofsensitivity of the MEMS microphone 1.

The supporting beam 7 has a thickness greater than that of the membrane2, for example, it is from three to five times as thick as the membrane2. In use, the variation of capacitance is caused principally by thedeformation of the membrane 2 and the relative displacement of thesupporting beam 7 (to which, as has been said, the membrane 2 isanchored) as compared to the rigid plate 3 is negligible.

The MEMS microphone 1 of FIG. 2 is provided according to the processdescribed in what follows and represented in FIGS. 3-8, only as regardsone half of the MEMS microphone 1 itself.

Initially (FIG. 3), a first insulating layer 15, for example, siliconoxide having a thickness of between 1 μm and 3 μm, preferably 2.6 μm, isgrown on a substrate 10, made, for example, of silicon of a thickness of500 μm, of a wafer 30. Then, a membrane layer 32, for example,polysilicon, is deposited on the wafer 30 on the first insulation-oxidelayer 15. Since, as is described more fully in what follows, themembrane layer 32 will form, at the end of the manufacturing steps, thesuspended membrane 2, its thickness must be carefully controlled duringthe step of deposition to obtain desired values of flexibility andmechanical solidity. For example, the membrane layer 32 can have athickness of between 0.5 μm and 1.5 μm, preferably 0.9 μm.

Then, the membrane layer 32 is doped by means of implantation of ionspecies of an N+ type, to increase the conductivity. A subsequentannealing step, for example, at a temperature of 1050° C. for 90minutes, favors diffusion and activation of the ion dopant species andthe reduction of the tensile stress of the membrane layer 32 at aninterface with the first insulating layer 15.

Next (FIG. 4), by means of successive lithography and etching steps, forexample, using a dry etch, the membrane layer 32 is selectively removed,with the exception of an area in which it is intended to form themembrane 2.

Then (FIG. 5), a second insulating layer 16, for example, TEOS(tetraethyl orthosilicate) or an oxide having a thickness of between 1μm and 2 μm, preferably 1.6 μm, is deposited on the wafer 30, densified,and then planarized. Subsequent lithography and etching steps (forexample, wet etching by means of basic oxide etch) of the firstinsulating layer 15 and of the second insulating layer 16 enableformation of an opening 35 for the ground contact 22.

Next, formed by dry etching are third openings 38 (only one of which isshown in FIG. 5) in the second insulating layer 16, until respectiveportions of a surface of the membrane layer 32 are reached and exposed.The third openings 38 enable, as described in what follows, formation ofthe supporting elements 6.

Then (FIG. 6), a rigid-plate layer 39 is formed on the wafer 30, to fillthe opening 35 and form a thick layer above it. Advantageously, therigid-plate layer 39 is constituted by epitaxial polysilicon, of athickness of between 4 μm and 8 μm, preferably 6 μm. The rigid-platelayer 39 is then planarized, for example, by means of chemicalmechanical polishing (CMP), to obtain a final thickness of between 3 μmand 7 μm, preferably 5 μm, whereas a subsequent step of implantation,activation, and diffusion of dopant species of an N+ type, for example,arsenic, enables an increase in the conductivity of the rigid-platelayer 39. The rigid-plate layer 39 is illustrated as merging with themembrane layer 32 to show the electrical communication established viathe supporting elements 6.

Then, a conductive layer, for example, made of aluminum with a thicknessof 0.7 μm, is deposited on the wafer 30 and defined by means of wetetching to form the first, second, and third pads 20, 21, and 23. In thefigure, by way of example, the pads 20, 21, 23 are placed on top of, andin direct contact with, the supporting beam 7 and the rigid plate 3.However, it may be advantageous to form the pads 20, 21, 23 in an areaof the substrate 10 not occupied by the supporting beam 7 and by therigid plate 3 (in a way not shown in the figures) and connect themelectrically to the latter via conductive paths. Next (FIG. 7), a firstmask layer 45, for example, made of SiC (silicon carbide), is depositedon a top surface of the wafer 30, while a second mask layer 47, forexample, made of SiN (silicon nitride) and/or SiC, is deposited on aback of the wafer 30.

As shown in FIG. 8, the first mask layer 45 is selectively removed toexpose portions of the rigid-plate layer 39. A subsequent etching step,for example, dry etching, enables removal of the portions of the exposedrigid-plate layer 39, but not the portions protected by the first masklayer 45. Defined in this way are the rigid plate 3, the supporting beam7, the holes 8 of the rigid plate 3 and of the supporting beam 7, thenotch 9, and the ground contact 22.

Then, the second mask layer 47 is selectively removed so as to leave aportion of the back of the substrate 10 underlying the membrane layer 32exposed. The substrate 10 is then removed, for example, by means of dryetching of a DRIE (deep reactive ion etching) type, in the portions notprotected by the second mask layer 47, to form the cavity 5.

Finally, a wet etch both on the front and on the back of the wafer 30enables removal of the portions of the first and second insulatinglayers 15 and 16 surrounding the membrane layer 32, thus leading toformation of the membrane 2, suspended above the cavity 5. The first andsecond mask layers 45, 47 are then removed, to form the MEMS microphone1 of FIG. 2.

It is clear that the manufacturing steps described can be used forproducing a plurality of MEMS microphones 1 on one and the same wafer30.

FIGS. 9-11 show a MEMS microphone 1 according to a further embodiment.In detail, FIG. 9 shows a top plan view of the MEMS microphone 1, FIG.10 a cross-sectional view of the MEMS microphone 1 along a cross sectionX-X of FIG. 9, and FIG. 11 a perspective view of the MEMS microphone 1,all according to said embodiment.

In this case, the MEMS microphone 1 comprises a single supporting beam7, including a beam-bearing portion 7 a anchored to the substrate 10 bymeans of two beam-anchorage portions 12, and four beam-appendageportions 7 b that start from the beam-bearing portion 7 a and areX-shaped in top plan view.

The supporting elements 6 are formed, in this case, each on a respectivebeam-appendage portion 7 b.

Advantageously, as may be seen in FIG. 11, the supporting elements 6 arecoupled to the membrane 2 through springs 51, fixed with respect to themembrane 2 and formed during the step of etching of the membrane layer32 (FIG. 4).

The springs 51, lying in the same plane in which the membrane 2 lies,can have, for example, an elongated cantilever shape with dimensions of50-200 μm in length L_(A), 20-40 μm in width L_(B), and with a thicknessequal to the thickness of the membrane 2. More precisely, the springs 51are made by notching the membrane 2. In this way, each spring 51 issurrounded by the membrane 2, from which it is partially separated by atrench 52, obtained using techniques of micromachining of a known type.

The springs 51 have the dual function of bestowing upon the membrane 2 agreater flexibility for movements orthogonal to the plane in which themembrane 2 itself lies, and further releasing the residual stresses ofthe membrane 2 deriving from the manufacturing process and fromanchorage to the supporting beam 7, thus enabling free radialcontractions or expansions (in the plane in which the membrane 2 lies)also in the proximity of the points of connection with the supportingelements 6, thanks to the presence of the trench 52.

In order to optimize release of residual stresses of the membrane 2 andthe stability of oscillation of the membrane 2 during use, the points ofanchorage of the supporting elements 6 to the membrane 2, via therespective spring 51, are preferably positioned at a distance from thecenter of the membrane 2 preferably equal to d/(2·√{square root over(2)}), where d is the diagonal of the membrane 2 (which joins twoopposite vertices of the membrane 2 in the case of the quadrangularmembrane shown in the figure).

FIGS. 12-14 show a MEMS microphone 1 according to another embodiment. Ingreater detail, FIG. 12 shows a top plan view of the MEMS microphone 1,FIG. 13 a cross-sectional view of the MEMS microphone 1 along a crosssection XIII-XIII of FIG. 12, and FIG. 14 is a perspective view of theMEMS microphone 1, all according to the present embodiment.

As shown in FIGS. 12-14, the MEMS microphone 1 comprises, according tothe present embodiment, a supporting beam 7 formed by a plurality ofbeam-bearing portions 7 a (for example four as shown in FIG. 12) and bya central beam portion 7 c. The beam-bearing portions 7 a depart, inpairs, on opposite sides of the central portion 7 c, and are anchored tothe substrate by means of respective beam-anchorage portions 12. Thecentral beam portion 7 c has, for example, a quadrangular shape, andmoreover carries a plurality of beam-appendage portions 7 b. In thefigures, four beam-appendage portions 7 b are shown, which extend eachon a respective side of the central beam portion 7 c and each of whichcomprises a supporting element 6 coupled to the membrane 2.

The plate-anchorage portions 11 of the rigid plate 3, according to thisembodiment, do not comprise the entire boundary of the rigid plate 3,but are provided by rigid-plate appendages 3 that are similar, asregards shape and dimensions, to the beam-bearing portions 7 a, albeitof smaller length than the latter. Furthermore, the points of anchorageto the substrate 10 of the plate-anchorage portions 11 and of thebeam-anchorage portions 12 are located at a short distance from oneanother. In this way, stresses external to the MEMS microphone 1 (forexample, if the MEMS microphone 1 is inserted within a package, adeflection of the package itself) will cause a deflection ordisplacement of the rigid plate 3 and of the supporting beam 7 (andhence of the membrane 2) of approximately equal amount, thus renderingthe functions of the MEMS microphone 1 less affected by this type ofexternal stress.

With reference to FIGS. 13 and 14, advantageously also in thisembodiment, the supporting elements 6 are anchored to the membrane 2through springs 51, fixed with respect to the membrane 2 and similar tothe springs 51 described with reference to FIGS. 10 and 11.

As already discussed with reference to the embodiment of FIG. 1, thefirst, second, and third pads 20, 21, and 23 are arranged by way ofexample.

In particular, shown in FIG. 12 are two second pads 21, each connectedto a respective rigid-plate region 3′, for biasing it and/or collectinga signal thereof during use. As an alternative, the two rigid-plateregions 3′ can be electrically connected to one another by means of aconductive path; in this latter case, just one second pad 21 would benecessary, electrically connected to just one of the two rigid-plateregions 3′.

FIG. 15 shows an electronic device 100 that uses one or more MEMSmicrophones 1 (just one MEMS microphone 1 is shown in the figure).

The electronic device 100 moreover comprises a microprocessor 101, amemory block 102, connected to the microprocessor 101, and aninput/output interface 103, for example, a keyboard and a video, alsoconnected to the microprocessor 101. The MEMS microphone 1 communicateswith the microprocessor 101 via a signal-treatment block 104, forexample, an amplifier. Furthermore, there may be present a loudspeaker106, for generating a sound on an audio output (not shown) of theelectronic device 100.

The electronic device 100 is preferably a mobile-communication device,such as a cell phone, a PDA, a notebook, a voice recorder, a reader ofaudio files with voice-recording capacity. Alternatively, the electronicdevice 100 can be a hydrophone, capable of working under water.

Finally, it is clear that modifications and variations may be made tothe MEMS microphone described and illustrated herein, without departingfrom the sphere of protection of the present disclosure.

For instance, the membrane 2 and the rigid plate 3 can have a shapedifferent from the quadrangular one; for example, they may have acircular or polygonal shape according to the design.

In the same way, the supporting beam 7 can have a shape different fromthe one shown and described with reference to FIGS. 1, 2, and 9-14. Forinstance, it may comprise just one beam-bearing portion 7 a having acantilever shape, anchored to the substrate by means of just onebeam-anchorage portion 12 or, instead, can comprise a plurality ofbeam-bearing portions 7 a, even separate from one another, and cancomprise any number of beam-appendage portions 7 b. The supportingelements 6 can be arranged in any way and in any number between thesupporting beam 7 and the membrane 2.

Furthermore, according to any embodiment, the membrane 2 can comprisesprings 51 of any shape and size, irrespective of the shape of thesupporting beam 7 and of the membrane 2.

Finally, it is clear that the rigid plate 3, the membrane 2, and thesupporting elements 6 can be made of any conductive material differentfrom doped polysilicon, for example, gold or aluminum.

The various embodiments described above can be combined to providefurther embodiments. All of the U.S. patents, U.S. patent applicationpublications, U.S. patent applications, foreign patents, foreign patentapplications and non-patent publications referred to in thisspecification and/or listed in the Application Data Sheet areincorporated herein by reference, in their entirety. Aspects of theembodiments can be modified, if necessary to employ concepts of thevarious patents, applications and publications to provide yet furtherembodiments.

These and other changes can be made to the embodiments in light of theabove-detailed description. In general, in the following claims, theterms used should not be construed to limit the claims to the specificembodiments disclosed in the specification and the claims, but should beconstrued to include all possible embodiments along with the full scopeof equivalents to which such claims are entitled. Accordingly, theclaims are not limited by the disclosure.

1. A process for manufacturing a MEMS acoustic transducer, comprising:providing a substrate; forming, on said substrate, a supportingstructure, fixed to said substrate; forming a membrane suspended aboveand fixed to said substrate through said supporting structure andconfigured to oscillate; forming a cavity in the substrate underneaththe membrane; and forming a plurality of anchorage elements arrangedbetween said membrane and said supporting structure, each anchorageelement being fixed to a respective portion of said membrane comprisedbetween a centroid and a perimetral edge of said membrane.
 2. Theprocess according to claim 1, further comprising forming a structurallayer and defining said structural layer so as to form said supportingstructure and an electrode electrically and mechanically separated fromsaid supporting structure.
 3. The process according to claim 2 whereinforming the membrane and the supporting structure further comprises:forming a first sacrificial layer on the substrate; forming a membranelayer on said first sacrificial layer; defining said membrane layer;forming a second sacrificial layer on said membrane layer; removingselective portions of said second sacrificial layer in areas that are toform the anchorage elements; depositing and defining said structurallayer on said second sacrificial layer; removing said first sacrificiallayer, so that said membrane is suspended above the substrate; andpartially removing said second sacrificial layer so that the electrodeand the supporting structure extend at a distance from the membrane. 4.The process according to claim 3 wherein the step of defining saidmembrane layer comprises forming a plurality of trenches to define aplurality of spring elements fixed to and surrounded by said membrane,in a position corresponding to the anchorage elements.
 5. The processaccording to claim 1 wherein the step of forming a cavity comprisesetching from a back said substrate to form an acoustic port facing saidmembrane.
 6. A method, comprising: forming a cavity in a substrate;forming a rigid electrode structure attached to the substrate; forming asupporting beam and anchorage portions from the rigid electrodestructure, the supporting beam and the anchorage portions beingmechanically and electrically separate; forming a membrane having aperimetral edge and a centroid that is suspended above the cavity andpositioned between the substrate and the rigid electrode; and couplingthe membrane to the supporting beam at locations that are between thecentroid and the perimetral edge.
 7. The method of claim 6 whereinforming the supporting beam includes forming a first pair of arms thatare transverse to a second pair of arms, each arm from the first pair ofarms being greater in length than each arm from the second pair of arms.8. The method of claim 6 wherein forming the supporting beam and theanchorage portions includes forming a first trench that separates afirst anchorage portion from the supporting beam and a second trenchthat separates a second anchorage from the supporting beam.
 9. Themethod of claim 6 wherein forming the supporting beam includes forming afirst pair of arms, a second pair of arms, and a third pair of arms, thesecond pair of arms being transverse to the first pair of arms, thethird pair of arms being transverse to the first pair of arms.
 10. Themethod of claim 9 wherein forming the first, second, and third pair ofarms includes forming each arm from the first pair of arms to be greaterin length than each arm from the second pair of arms and greater inlength than each arm from the third pair of arms.
 11. The method ofclaim 6 wherein coupling the membrane to the supporting beam atlocations that are between the centroid and the perimetral edgeincludes: forming a plurality of spring elements from the membrane;coupling each spring element to the supporting beam.
 12. The method ofclaim 11 wherein forming the plurality of spring elements includesforming a plurality of openings in the membrane that partially separatethe spring elements from the membrane.
 13. The method of claim 6 whereincoupling the membrane to the supporting beam at locations that arebetween the centroid and the perimetral edge includes removing portionsof a single layer.
 14. A method, comprising: forming a membrane layer ona substrate; forming a rigid electrode layer on the membrane layer;forming a supporting member, a first anchor member, and a second anchormember from the rigid electrode layer, forming the supporting memberincluding; separating the supporting member from the first and secondanchor member by removing portions of the rigid electrode layer; forminga moveable membrane, the forming of the membrane including: forming aplurality of supporting spring elements in the membrane by removingportions of the membrane layer; and coupling the supporting member tothe plurality of spring elements.
 15. The method of claim 14 whereincoupling the supporting member to the plurality of spring elementsincludes partially separating the spring elements from the membrane witha plurality of openings.
 16. The method of claim 14 wherein forming theplurality of spring elements includes forming a plurality of cantileverbeams having a moveable end that faces a center of the membrane.
 17. Themethod of claim 16 wherein forming the plurality of spring elementsincludes forming a plurality of U-shaped openings around the cantileverbeams.
 18. A method, comprising: forming a cavity in a substrate;forming a membrane above the cavity, the membrane having a center and aperimeter edge; forming a plurality of spring elements that are coplanarwith the membrane; forming a trench partially separating the pluralityof spring elements from the membrane; and forming a supporting structurehaving a first anchorages coupled to the substrate and second anchoragescoupled to the plurality of spring elements at locations between thecenter and the perimeter edge.
 19. The method of claim 18 whereinforming the supporting structure includes forming the first anchoragesof the supporting structure at locations spaced from the center beyondthe perimeter edge.
 20. The method of claim 18 further comprisingforming a rigid plate coupled to the substrate, the rigid plate having afirst part separated from a second part by the supporting structure.